A tungsten oxide thin film is widely used as a dielectric material used in capacitors, filters, electronic devices such as semiconductor chips, and the like, as an optical element material used in optical communication filters, isolators, and the like, as an electrochromic material used in light control mirrors and the like, and as a gaschromic material used in gas sensors and the like. It is also known that tungsten oxide functions as a visible-light responsive photocatalytic material, and it is a material drawing great attention in view of industrial applicability. Further, tungsten oxide microparticles have an excellent function as a visible-light responsive photocatalyst, and a film containing the tungsten oxide microparticles are drawing attention.
A photocatalytic film using tungsten oxide is formed by, for example, applying a dispersion containing tungsten oxide microparticles on a surface of a base material of a product to which photocatalytic performance is to be imparted. It is known an aqueous dispersion which contains tungsten oxide microparticles having an average primary particle size (D50 particle size) in a range of from 1 to 400 nm and whose pH is in a range of from 1.5 to 6.5. According to such an aqueous dispersion, dispersibility of the tungsten oxide microparticles is enhanced and formability of a film containing the tungsten oxide microparticles is improved. Therefore, when the film which is formed by applying the aqueous dispersion containing the tungsten oxide microparticles on the base material is used as a photocatalytic film, visible-light responsive catalytic performance of the tungsten oxide microparticles can be exhibited.
A conventional photocatalytic film containing tungsten oxide microparticles exhibits a 5% gas decomposition rate or more under an environment where illuminance of visible light is about 2000 lx. However, considering practicality of the photocatalytic film, decomposition performance for harmful gas such as acetaldehyde is not sufficient, which has given rise to a demand for improvement of the gas decomposition performance. Further, being poor in gas adsorbing power, the conventional photocatalytic film has a problem that its gas decomposition speed becomes low under an environment with a low gas concentration. Such circumstances have given rise to a demand for an increase of the gas decomposition performance by a visible-light responsive photocatalyst.